The present application generally relates to plasma arc torch systems, and more particularly relates to a plasma arc torch system having the capability both to cut (i.e., sever or divide into multiple pieces) and to mark (i.e., produce visible lines on the surface of) metal workpieces without having to change any of the consumables of the torch.
Dedicated plasma marking torches are commonly employed to mark metal plates and other workpieces with lines, numbers, or symbols to facilitate subsequent plate layout and assembly operations. Also, alpha-numeric markings are sometimes employed for plate identification purposes. Known marking torches usually operate at low amperage (e.g., about 8-10 amperes) and employ a non-cutting gas, such as argon, as the plasma gas, which forms a plasma stream that exits through the nozzle orifice and forms a visible mark on the workpiece. Known marking torches also typically include an air cooling system that circulates air into contact with the electrode and then along the outside surface of the plasma nozzle so as to be discharged coaxially about the plasma stream.
It would be desirable to have a plasma arc torch that is able to both mark and cut workpieces, and perform both tasks with high precision. The assignee of the present application has previously developed a plasma arc torch having both marking and cutting capabilities, as described in U.S. Pat. No. 6,054,669, the entire disclosure of which is incorporated herein by reference. The torch described in the '669 patent comprises a torch body defining a longitudinal axis, and an electrode mounted to the torch body along the longitudinal axis and defining a front discharge end. A nozzle is mounted on the torch body to overlie the front discharge end of the electrode and so as to define a plasma cavity therebetween, and the nozzle includes a front wall having a bore therethrough that is aligned with the electrode along the longitudinal axis. A shield is mounted to the torch body so as to overlie in spaced relation the front wall of the nozzle and define a gap therebetween, which gap forms an annular orifice that coaxially surrounds the bore of the nozzle. A plasma gas passage extends through the torch body and to the plasma cavity, and a plasma gas control is provided for delivering a plasma marking gas to the plasma gas passage and thus to the plasma cavity. Also, a shield gas passage extends through the torch body and to the annular gas orifice, and a shield gas control is provided for delivering a gas (optionally also containing a water mist) to the shield gas passage and thus to the annular gas orifice. A power supply is also provided for delivering electrical power to the electrode at a relatively low power level that is suitable for plasma marking of a workpiece.
The orifice of the nozzle for the above-described torch was quite small in diameter, on the order of 0.018 to 0.043 inch. Accordingly, the arc density even at the relatively low marking arc current level (10 to 35 amperes) was very high. This enabled the torch to form high-quality marks on carbon steel and stainless steel. However, in order to form such high-quality marks, the marking speed (i.e., linear speed of the torch, typically measured in inches per minute, or “ipm”) had to be relatively low.
In some applications (e.g., marking of alignment marks on large plates such as for shipbuilding or the like), it is desired to be able to mark at much higher marking speeds. In order to be able to increase the marking speed with the above-described torch, it is necessary to increase the arc current. Alternatively, it was found that adding the optional water mist in the shield gas allowed a modest increase in marking speed without increasing the arc current. However, it was still not possible to mark at speeds exceeding about 300 ipm even when the water mist was employed, and the highest attainable speed for acceptable-quality alphanumeric marking was about 100 ipm.